1. Field of the Invention
This invention relates to a molding process for continuously carrying out injection molding of a preform formed of a synthetic resin and stretch blow molding into a hollow molded article of which major portion such as a body portion is ultra-thin.
2. Background Art
As one of molding processes referred typically to as injection stretch blow molding, there is the process that comprises holding an injection molded preform at a mouth portion thereof with a lip mold and transferring it immediately to a blow mold to carry out stretch blow molding.
A 3-station type molding process described in Japanese Patent Laid-open Publication No. 4-214322 or European Patent Publication No.454997A1 comprises injecting a molten resin into an injection cavity to form a desired preform, releasing the preform from the injection mold with a mouth portion thereof being held with a lip mold while it is in a state where a skin layer generated on the surface of the preform as a result of quick cooling enables one to keep the configuration and at a high temperature state where interior cooling is not completed, transferring it from the injection mold to the blow mold by using the lip mold as it is, and stretch blow molding the preform into a thin hollow molded article in a time duration before the surface temperature of the preform that rises due to its own interior heat reaches a peak temperature.
This molding process is remarkably effective to stretch blow mold a molded article such as a bottle comprising a body portion having an average thickness of 0.2.about.0.35 mm, but is considered to cause trouble to stretch blow mold an ultra-thin molded article (having a 0.15 mm or thinner body portion) at a higher stretching ratio.
A thick and short preform is used for the stretch blow molding of a molded article that requires a high stretching ratio. However, when using polyethylene terephthalate or the like, it takes time to cool the preform through the injection mold when the preform has a larger thickness and crystallization of the preform causes whitening. In cases of a cold parison technique in which a preform cooled to a room temperature is re-heated for the stretch blow molding or a temperature controlling technique in which a preform at a high temperature is further heated to control the temperature thereof before the stretch blow molding, this crystallization does not particularly cause a problem because the heating is immediately before the stretch blowing, so that the thickness may be determined up to 4.0 mm. However, in a process where the preform released from the mold at a high temperature is immediately subjected to the stretch blow molding, the thickness is limited to 3.0 mm due to great influence of the crystallization. It is therefore extremely difficult to increase the stretching ratio for a thick short preform.
Furthermore, in the process where the preform released from the mold at a high temperature is immediately subjected to the stretch blow molding, the molding temperature is lower than that in the molding processes using the cold parison technique or temperature controlling technique because the stretch blow molding is carried out with the surface temperature of the preform being increased by its own interior heat to higher than the glass transition temperature (Tg), and just when the preform is stretch blow molded, the temperature of the preform decreases rapidly due to the increase in surface area as a result of stretch expansion. Accordingly, thickness deviation or crazing tends to occur to form a bad shape when the stretching ratio is such that the temperature in the last phase of the stretch expansion is significantly lower than the glass transition temperature.
This molding temperature may be 95.degree. C. or higher by means of reducing the cooling time to control the surface temperature immediately after the releasing from the mold to 70.degree. C. or higher. Nevertheless the peak temperature does not reach 100.degree. C. The quantity of internal heat is insufficient at 100.degree. C. or lower and is not enough to stretch and expand the preform by 13 times or more.
It is considered in stretch blow molding that the thickness of the major portion of, for example, a bottle can be reduced by means of increasing the stretching ratio depending on the thickness and the temperature of the injection molded preform. This reduction in thickness is greatly useful for savings in materials and reduction of wastes and the effects thereof are great both in economically and socially terms. The thickness reduction greatly deteriorates the buckling strength and thus there is a limitation of use as a single body, but it may be developed as a novel packaging container when combined with a paper container or the like. On the other hand, it is difficult, in the molding process in which the injection molded preform is immediately subjected to the stretch blow molding, to increase the stretching ratio to form the bottle to have an ultra-thin major portion because of the combination of the above mentioned limitation on the thickness of the preform and the molding temperature.
Therefore, an object of this invention is to provide a novel process for injection stretch blow molding that allows production of hollow molded articles having a thickness so ultra-thin that it can be folded, without increasing the thickness of the preform beyond that of conventional ones, even in the process in which the preform released from the mold at a high temperature is immediately subjected to the stretch blow molding.